signed or unsigned integers using %v or the formatless scanner.
That is, Sscan("0x11", &i) or Sscanf("0x11", "%v", &i) will now
set i to 17.   If a format other than %v is presented, the behavior
is as before.

Fixes #1469.

R=rsc
CC=golang-dev
https://golang.org/cl/4131042